The invention relates to a coordinate measuring apparatus having a probe movable in the coordinate directions for scanning a workpiece. The coordinate measuring apparatus also includes a mechanism with drives via which the probe is moved as well as a control apparatus for controlling the mechanism.
Coordinate measuring apparatus of this kind have long been known and are offered in various configurations. For example, a coordinate measuring apparatus can be of the so-called portal type having a portal mechanism wherein a portal spans a measuring table. The portal is movably guided on the measuring table in a first coordinate direction and can be moved via corresponding drives, such as friction wheel drives or spindle drives. A so-called transverse sled is mounted in the spanning part of the portal and can likewise be moved by drives in a second coordinate direction along the spanning part of the portal. On the transverse sled, in turn, a vertically aligned measuring arm is guided in the third coordinate direction. A probe is disposed at the lower end of the measuring arm and, with the probe, a workpiece to be measured can be scanned. The probes can be contacting probes which mechanically touch the workpiece and wherein the contact touching is detected via a deflection of the probe or, for example, the probes can be optical probes with which the surface of the workpiece to be measured is optically scanned.
Another relatively widely distributed configuration of coordinate measuring apparatus is the so-called stand measuring apparatus having a stand mechanism wherein a vertically aligned column is movably guided along a measuring table and which can be displaced via drives. A so-called cross slide is movably guided on the column in the vertical direction. On the cross slide, a horizontally guided measuring arm is movably guided and the probe is disposed on the end of the this measuring arm. This coordinate measuring apparatus is likewise driven by corresponding drives.
Control units are usually provided for controlling such coordinate measuring apparatus. The control units comprise, for example, an input computer and an evaluation computer and a separate control arrangement wherein the hardware functions for controlling the mechanics of the coordinate measuring apparatus are realized. The traveling paths and the evaluation requirements are usually defined in the computer. The travel paths are especially driven along for scanning the workpiece. For specifically defining the traveling paths, so-called geometric elements are defined based on which the geometry of the workpiece to be measured is simulated.
The geometric elements define the smallest possible definable geometric units based upon which the geometry, which is actually to be scanned, of the workpiece to be measured is simulated. On the basis of this geometry, the actual points, which are to be scanned on the surface of the workpiece, are defined. A usual compilation of such geometric elements are: the point, the circle, the cylinder, the cone, the sphere, the ellipse, the plane, the straight line, the torus, and the paraboloid as well as the curve and the free-form surface.
The points, which are to be scanned on the workpiece surface, are defined with this compilation of geometric elements in coordinate measuring apparatus.
In coordinate measuring apparatus known up to now, the geometric element as well as the points, which are to be scanned on the geometric elements, were always referred to a common reference coordinate system, for example, the machine coordinate system of the coordinate measuring apparatus or the workpiece coordinate system of the workpiece to be measured. Such a reference to a common coordinate system is absolutely necessary during the execution of the measuring sequence because, otherwise, the probe could not be correspondingly moved.
The storage of the points to be scanned relative to the reference coordinate system had, however, the peculiarity that each time there was a change of the geometry of the workpiece and therefore also of the geometric elements, the points, which were to be scanned hereon, had to be completely newly defined. However, often small changes on the geometry of the workpiece to be measured are undertaken in different product stages or in components of a family of products. For this reason, this leads to a high complexity with respect to programming and this is especially so when the points to be scanned on the particular geometric element must be manually inputted with tedious programming work.
It is an object of the invention to provide a coordinate measuring apparatus proceeding from the above as well as a corresponding method for controlling such coordinate measuring apparatus with which the travel paths can be more easily changed.
The basic idea of the invention is that the parameters for defining the geometric elements and the points, which are to be scanned on the geometric elements, are stored referred to a coordinate system corresponding to the particular geometric element. In this way, the individual points of the geometric element to be scanned are in a fixed relationship to the geometric element coordinate system so that the desired geometry of the workpiece can easily be changed.
In order to clearly define the geometric element coordinate system, which is inherent to the geometric element, parameters (additionally assigned to each geometric element) are stored in the memory of the control unit as to offset of the geometric element coordinate system and/or parameters as to the orientation of the geometric element coordinate system with respect to a common reference coordinate system. In this way, each geometric element as such, referred to a common reference coordinate system, receives a clear offset and direction. In this way, a once defined geometric element can, together with the points to be scanned, be positioned and rotated without difficulty at various locations on the workpiece without having to newly input the geometric element and the points to be scanned. With a position change of the geometric element of this kind, only the parameters of the offset of the geometric element coordinate system and/or the orientation with respect to the common reference coordinate system are changed.
A special problem results additionally for some geometric elements when the dimension of the geometric element is changed because, for a corresponding change of the dimensions, the points, which are to be scanned, possibly no longer lie on the surface or the line of the geometric element. For example, if the diameter of a circle is changed, then the points, which are to be scanned, no longer lie on the circular line after the change without corresponding measures. According to an especially advantageous embodiment of the method of the invention, and for a change of the dimensions of a geometric element, the parameters with respect to the points, which are to be scanned on the geometric element, are changed in such a manner that the perpendicular spacing of each point, which is to be scanned, from the geometric element, remains constant.